1. Field of the Invention
The present invention relates generally to an optical direct amplifier device. More particularly, the invention relates to a monitoring circuit of an optical direct amplifier device.
2. Description of the Related Art
In the conventional optical direct amplifier device as illustrated in FIG. 5, a rare-earth added fiber 1a, a WDM (Wavelength Division Multiplexing) coupler 3a, an optical isolator 4a, an optical wavelength multiplexer/demultiplexer 5a are connected in sequential order from an uplink signal input end to reach an uplink signal output end. Similarly, a rare-earth added fiber 1b, a WDM coupler 3b, an optical isolator 4b, and an optical wavelength multiplexer/demultiplexer 5b are connected in sequential order from a downlink signal input end to reach a downlink output end.
Laser diodes (LD) 6a and 6b are connected to a control circuit B so that outputs thereof are controlled by the control circuit 8. The laser diodes 6a and 6b are connected to two input ends of the optical wavelength multiplexer/demultiplexer 7. Two output ends of the optical wavelength multiplexer/demultiplexer 7 are connected to input ends, other than input/output paths, of the WDM couplers 3a and 3b, respectively.
On the other hand, two branched ports of the optical wavelength multiplexer/demultiplexer 5a are connected to respective optical wavelength multiplexers/demultiplexers 10a and 10b. Also, two branched ports of the optical wavelength multiplexer/demultiplexer 5b are connected to respective optical wavelength multiplexers/demultiplexers 10a and 10b. 
And, branched ports of optical wavelength multiplexers/demultiplexers 10a and 10b are connected with each other. In an up-link circuit and a down-link circuit of the foregoing optical direct amplifier, connecting portions of the optical wavelength multiplexers/demultiplexers 5a and 5b located downstream side of the optical isolators 4a and 4b form the monitoring circuit.
Monitoring of the optical direct amplifier device in an optical direct amplifier system employs a method to transmit a monitoring signal from a terminal station and returning a part of the optical signal within the optical direct amplifier device to the opposite side line (e.g. from the up-link line to the down-link line) and to receive the monitoring signal turned back and returned by the terminal station. In general, there has been employed a method for detecting in the optical direct amplifier device having an abnormality of optical output based on a lower magnitude of the returned monitoring signal. However, in the monitoring circuit of the conventional optical direct amplifier device, when one of the laser diodes 6a and 6b in FIG. 5 is turned OFF, lowering the magnitude of the monitoring signal, output of the optical direct amplifier device can be small causing difficulty in detecting an abnormality of optical output in the optical direct amplifier device with certain characteristics of the rare earth added fibers 1a and 1b or certain monitoring signal wavelength.
The present invention has been Worked out in view of the problem set forth above. Therefore, it is an object of the present invention to provide an optical direct amplifier device which can detect failure of an excitation light source without using an electric circuit, even when a monitoring system of the optical direct amplifier device based on a line turning back is employed.
According to one aspect of the present invention, an optical direct amplifier device including first and second optical amplifiers respectively amplifying signal lights of first and second transmission path, comprises:
a monitoring circuit branching only monitoring light of a wavelength different from a wavelength of the signal lights; and
extracting means for extracting a part of an excitation light incident on the optical amplifiers.
Namely, the optical direct amplifier device of the present invention is characterized by a capability of performing failure detection of the excitation light source of the high precision optical direct amplifier device in the optical direct amplifier system.
In the preferred construction, each of first and second optical amplifiers which respectively amplifying signal lights of first and second transmission paths is formed of a series circuit of a rare earth added fiber, a wavelength division multiplexing coupler, an optical isolator and an optical wavelength multiplexer/demultiplexer, and further includes a monitoring circuit for branching only monitoring light of a wavelength different from the wavelength of the signal light and combining the branched light in the other transmission path, the optical wavelength multiplexer/demultiplexer extracting a part of the excitation light incident on the rare earth added fiber.
In the optical direct amplifier device according to the present invention, the rare earth added fiber amplifying the monitoring light is provided in the monitoring circuit. The excitation light incident on the rare earth added fiber is the excitation light extracted by the optical wavelength multiplexer/demultiplexer. The optical direct amplifier device includes a wavelength division multiplexer for combining the excitation light with the rare earth added fiber.
When one of two laser diodes fails, an excitation light power incident on the rare earth added fiber is reduced to make the loss of the rare earth added fiber greater than the normal state and thus to make lowering magnitude of the monitoring signal output of the optical direct amplifier device in the monitoring signal wavelength greater.
With the construction and operation set forth above, in the optical direct amplifier system employing the monitoring system of the optical direct amplifier device based on turning back of the line, failure of the excitation light source of the optical direct amplifier device can be detected with high precision without using an electric circuit.
The monitoring circuit may include optical amplifying means for taking a part of excitation light for the first and second optical amplifiers extracted by the extracting means as an excitation light source, and amplifying the monitoring optical wavelength.
The optical direct amplifier device may further comprise means for coupling a part of the excitation light for the first and second optical amplifiers extracted by the extracting means, with the optical amplifying means. The optical amplifying means may be set to have a gain in normal state of the excitation light source at less than or equal to 0 dB.
The extracting means may use the excitation light leaking from an extra port of a coupler for multiplexing the excitation light of the first and second optical amplifiers.
The monitoring circuit may be constructed by connecting one of ports of a selective wavelength reflector to selectively reflect the monitoring light wavelength to the optical amplifying means, and connecting the other port of the selective wavelength reflector to the extracting means.
The optical direct amplifier device may further comprise means provided downstream of the selective wavelength reflector for connecting one of the ports to the optical amplifier means and attenuating a signal other than the monitoring light wavelength passing through the selective wavelength reflector. One light emitting element of the excitation light for the first and second optical amplifiers may be provided for each of the first and second transmission paths, and a part of the excitation light of the light emitting element is supplied for each of the light amplifying means for the first and second transmission paths. In the alternative, a plurality of the light emitting elements of the excitation lights for the first and second optical amplifiers may be provided for each of the first and second transmission paths, and parts of the excitation lights of the light emitting elements are supplied in plural for each of light amplifying means for the first and second transmission paths. In the further alternative, one light emitting element of the excitation light for the first and second optical amplifiers may be provided for each of the first and second transmission paths, and a second wavelength division multiplexer is provided in each of first and second transmission paths, a part of excitation light of the light emitting element and a signal of the branched monitoring light wavelength are combined by the second wavelength division multiplexer to supply an output thereof to the optical amplifier means.